1. Field of the Invention
This invention relates to an evaporative emission control system for internal combustion engines, which prevents emission of evaporative fuel generated in a fuel tank of the engine into the atmosphere.
2. Prior Art
An evaporative emission control system of this kind has been proposed, e.g. by Japanese Provisional Patent Publication (Kokai) No. 1-159455, which includes not only an ordinary canister which adsorbs evaporative fuel generated in a fuel tank of the engine during parking of a vehicle in which the engine is installed or during operation of the engine, but also a canister for exclusive use at refueling, which adsorbs evaporative fuel generated during refueling into the fuel tank. According to the proposed evaporative emission control system, to overcome the disadvantage with conventional canisters that they have an insufficient adsorbing efficiency due to a high flow velocity of evaporative fuel generated during refueling, the canister for exclusive use at refueling has a plurality of layers of adsorbents formed of activated carbon defined therein by one or more partitions, wherein the flow path of evaporative fuel is deflected so as to enhance the adsorbing efficiency without increasing the size (ratio L/D (length/diameter)) of the canister.
Further, it is known that if a large amount of HC molecules, as one of component elements of fuel, remain adsorbed by an adsorbent in the canister, an equilibrium adsorption phenomenon occurs in the canister, which is caused by movement of HC molecules such that the concentration of HC molecules becomes homogeneous throughout the canister, according to a change in the temperature of the canister with the lapse of time. To prevent this equilibrium adsorption phenomenon, an evaporative emission control system has been proposed, which includes a canister, the interior of which is divided into two activated carbon chambers, one for use in purging/charging, and the other for use in draining into the atmosphere, and a passage communicating between the two chambers, such that the interior of the canister has a generally U-shaped construction. In addition, this proposed evaporative emission control system is designed such that the volumetric size of the activated carbon chamber for purging/charging is larger than that of the activated carbon chamber for draining, whereby, during purging, the concentration of HC within the activated carbon chamber for purging/charging is reduced to a value lower than that of HC concentration within the activated carbon chamber for draining. As a result, occurrence of the equilibrium adsorption phenomenon can be restrained, leading to effective adsorption of the HC component to the activated carbon and hence preventing the HC component from passing through the canister without being adsorbed thereby.
In the former proposed evaporative emission control system, however, the canister for exclusive use at refueling is not used on occasions other than refueling, and becomes empty after evaporative fuel adsorbed therein is discharged or purged into the engine. Thus, the utilization factor of the activated carbon is low. Further, two batches of activated carbon have to be provided as adsorbents, one for the canister for exclusive use at refueling, and the other for the ordinary operation, leading to use of a large amount of activated carbon and hence an increased cost. Moreover, at least two purging passages connecting between the respective canisters and the intake passage of the engine have to be provided to discharge evaporative fuel adsorbed by the canisters, resulting in that the evaporative emission control system has a complicated construction.
On the other hand, in the latter proposed evaporative emission control system, since evaporative fuel (HC component) is generated in a large amount during refueling, it is impossible for the single canister to adsorb evaporative fuel generated during refueling and during stoppage of the engine. Therefore, a canister with a further improved adsorbing capacity is desired.